In the field of optical and magneto-optical computer information storage systems, it has long been recognized that incorporating physical features into the surface of a storage element, such as a disc or card (hereafter referred to as “media” or “medium”), provides a number of advantages for data storage media. Precise position and tracking, error correction, focusing, and other information can be provided or enhanced by these surface features, and this information is used by the hardware and control system with which the storage element is designed to operate (hereafter referred to as “drive” or “transport”). These surface features are “read” by means of an optical pickup device (hereafter referred to as “optical head” or “optical pickup unit”) that is a key component of the drive. Media surface features typically include pits, lands, grooves, and the like. For the majority of optical storage media, the surface features are incorporated into the media (e.g., the disc substrate) at the time of manufacture, and this process is generally referred to as physical pre-formatting (herein “pre-formatting”).
In the case of recordable and erasable compact discs (“CD”), digital versatile discs (“DVD”), magneto-optical discs (“MO”), and other media, such pre-formatting is accomplished by means of a molding process, whereby a molten polymer (substrate) material is brought into contacted with a patterning surface (“tool”) whose surface contains the mirror-image of a surface relief structure that is to be imparted to the disc surface. For example, U.S. Pat. No. 4,428,069 shows one such method for pre-formatting discs. After sufficient cooling has occurred, the disc is removed from the molding machine, and various layers are applied over this surface relief structure, such as reflective layers, recordable layers, protective layers and the like.
A CD typically has a single spiral track of data, circling from the inside of the disc to the outside of the disc. The spiral track has very fine surface modulations (often in the form of pits, bumps, or grooves) containing features with dimensions in the submicron size range. When a CD is played, a laser beam passes through the CD's polycarbonate substrate layer, reflects off a reflective layer to an optoelectronic device that detects changes in light. The difference in height of the pits, bumps, and grooves relative to the flat parts of the substrate surface results in a change, or modulation, of the reflected light. An optoelectronic sensor in the head detects these changes in reflectivity, and the electronics in the CD-player (drive) interpret the changes as data bits. For pre-recorded information (music, software, etc.), these pits are used to store the data, as well as provide positional information. For recordable or erasable discs, the pre-formatted structures are typically used for positioning, tracking, and writing/erasing user data.
In the present art, a durable tool, often referred to as a “stamper”, is used to impart the pattern into the substrate surface and is typically made from a “master” pattern by a metal electroforming or electroless plating process. The master pattern, in turn, is made on a laser beam recorder, a device in which a recording medium, consisting of a photosensitive layer coated on a substrate is rotated on a lathe or spindle and exposed to a modulated laser beam. Chemical development of the exposed pattern results in a surface relief pattern that will ultimately be replicated into the optical disc substrate, as previously described. Although a number of variants exist, such steps as these are typical of the basic manufacturing process of optical discs.
The performance and tolerance requirements of the laser beam recorder systems that create the master patterns are very high and, therefore, the process requires very expensive hardware and optical components, and the laser beam recorder systems must be housed in a clean-room environment. The molding process used to make the polymer substrates mechanically reproduces the master pattern. It should be noted that the relief structures that are molded into the surface of optical storage media are very precise copies of the same features that the laser beam recorder laser inscribes into the master substrate.
The manufacturing process described above dominates the optical disc manufacturing industry and is designed to enable very low-cost media and hardware production. Low-cost production is achieved by placing the requirements for high precision and accuracy in the master pattern step, which is done relatively infrequently. Precision molding is used to make the plastic replicas rapidly and inexpensively and with nearly the same level of precision and accuracy as the original master pattern, as noted above. This approach has enabled the production of low cost discs in high volumes, and for this reason, the process of pre-molding the surface features, for both pre-recorded and recordable/erasable optical discs, has completely replaced early variants in which formatting was incorporated either after the disc was manufactured or “in the field”.
The accuracy, precision, and small feature size that can be achieved in a laser beam recorder mastering facility is greater than can be achieved by carrying out this operation in the field, since the relatively inexpensive drives used by industrial an/or consumer optical disc systems do not have the same level of precision as the laser beam recorders used to create the master pattern. The higher information density (i.e., closer and smaller features) achievable by a laser beam recorder, relative to an inexpensive drive, allows more information to be stored on a disc, so thus pre-formatted optical discs have a much higher areal density (measure of the number of bits stored per area) than discs in which such features were written by means of an inexpensive drive with lower resolution capabilities. Accordingly, it is commonly recognized that the low cost and high capacity of today's optical storage discs would not be possible without pre-formatting.
For purposes of the present disclosure, it is also useful to compare the characteristics of the aforementioned optical disc systems to magnetic tape, which is another common form of removable information storage. Magnetic tape recording systems utilize tape media that typically ranges in size from 4 mm to 35 mm in width, and from tens of meters to thousands of meters in length. Magnetic tape is available in a number of physical storage configurations, including open reel, single hub cartridge, and dual spool cassette. Magnetic tape characteristically provides a very large amount of surface area for storing information. By way of comparison, the tape in a typical 120 minute video home system (“VHS”) tape cartridge has roughly 250 times more usable surface area than a CD.
In addition to their respective advantages, optical disc and magnetic tape removable information storage systems also suffer from a number of limitations. Disc-based systems, although characteristically having a significantly higher areal density than magnetic tape, are limited by the total available surface area. A number of variations of the basic optical disc exist or have been proposed for overcoming this limitation, including use of multiple layers, multiple sides, gray-scale (multi-level) recording, near-field, fluorescent multi-layers, holographic, to name but a few. These variants of the optical disc, however, only increase the effective surface area by a factor of about 2 to 20 over the basic optical disc design.
Magnetic tape, while having significantly greater surface area than optical discs, suffers from lower areal density. Although very high data density has been achieved with magnetic hard disk systems, the storage density of magnetic tape has lagged behind hard disks by many orders of magnitude. The lower areal density is due to the intrinsic difficulty in controlling the magnetic tape head-media interface as precisely as can be achieved in hard disk systems.
In addition, magnetic tape systems are susceptible to mechanical wear to both magnetic head and media because of the necessary head-media contact and the intrinsic abrasiveness of magnetic media. Some magnetic tape media are also characterized by a limited storage and operational lifetime resulting from degradation of the magnetic media over time.
It would appear useful, therefore, to combine the beneficial aspects of magnetic tape (linear media with a large storage surface area) and optical recording (high areal density and a longer operational lifetime) in an “optical tape”. To date, only one such system has been commercialized. This optical tape system is disclosed in U.S. Pat. Nos. 4,567,585 and 5,177,724, and was commercially available from CREO Products of Vancouver Canada. The CREO optical tape system, however, was physically large and very expensive (i.e., $250,000). The CREO optical tape system used 12-inch open reel spools of 35 mm optical tape, which hold 1 Terabyte of data (and initially sold for $10,000 per spool). The tape consists of a dye-polymer-based media developed by ICI ImageData, a subsidiary of ICI (Imperial Chemical Industries of Great Britain), and disclosed in U.S. Pat. No. 5,382,463. This system was not a commercial success and only several dozen units were ever sold. Other optical tape systems have been disclosed in U.S. Pat. Nos. 5,784,168, 5,825,740, 5,802,033, 5,581,534, 5,734,539, 5,120,136, and 6,141,301.
A serious drawback with the previous attempts to carry out optical or magneto-optical recording in a tape format lies in the optical head/media design. Virtually all of the previously-mentioned systems were based on optical head technologies typically built around proprietary single or multi-channel optical read/write head architectures (such as those disclosed in U.S. Pat. Nos. 5,097,457, 4,661,941, 5,673,245, and 4,884,260), with unformatted tape media (such as those disclosed in U.S. Pat. Nos. 5,234,803, 5,382,463, 5,358,759, 5,459,019, 4,904,577, 4,960,680, 5,015,548, 5,196,294, 5,465,241, 5,358,759), all of which rely upon complex and custom optical head designs. These optical tape systems use a variety of read/write technologies, including vertical cavity surface-emitting lasers (“VCSEL”) based arrays, magnetically levitated spinning polygons, and multiplexed high-power lasers with custom semiconductor channel modulators. These systems are all based on expensive and/or complex optical head architectures, which considerably increase the cost and development time for such systems. Additional drawbacks to these systems include one or more of the following: the inability of fixed position multiple beam heads to deal with large track pitch variations (e.g., resulting from dimensional changes in the tape substrate), the potential cost and difficulty of replacing one or more head elements when it malfunctions or fails, the difficulty and precision required to align individual head elements in a multi-beam system, especially in the field.
There have been various proposals for dealing with some individual aspects of these problems (such as those disclosed in U.S. Pat. Nos. 5,239,528, 5,120,136, and 4,633,455). For example, an optical tape drive “including redundant optical heads to continue reading and writing data to an optical tape in the event of failure of one or more optical heads” is disclosed in U.S. Pat. No. 6,058,092. But no proposed solution or previous art addresses an integrated system, including the media and the head, that solves all of these problems and disadvantages of the prior art.
What is still desired is a new and improved optical tape system that provides the benefits of practical, low-cost pre-formatted optical disc media used with low cost commercially available optical heads, and provides high areal density and a longer operational lifetime. The new and improved optical tape system will also include the beneficial aspects of a linear media with a large storage surface area.